In drilling oil and gas wells, a conventional practice is to take samples of the strata through which the drill bit is passing. By analyzing these samples with respect to such parameters as permeability, porosity and fluid saturation, a great deal can be learned regarding the nature of the particular strata from which the sample was taken. These tests are generally included in the generic term "core analysis" which allows the characteristics of a particular reservoir to be evaluated.
When a well is drilled into a permeable formation, a drilling mud is circulated in the well to counterbalance the pressure of oil, gas, etc. A typical drilling mud consists of a base fluid of oil or water in which fine-grained mineral matter is suspended. The drilling mud may enter the formation and displace the connate fluids such as brine and hydrocarbons. The extent and nature of such drilling mud invasion is dependent upon various factors and generally is unpredictable. This is of particular consequence in the area of core sample analysis wherein various calculations, such as the calculation of native oil and water saturation, may be significantly affected by the amount of drilling mud invasion. These calculations may be made more accurately having knowledge that substantial amounts of drilling mud and/or drilling mud filtrate are present in the core sample.
Different methods of identifying drilling mud invasion of a core sample are known. Some of these methods involve the use of computed tomography (CT). Computed tomography is a technology that provides an image of the internal structure of a cross section or slice through an object via the reconstruction of a matrix of X-ray attenuation coefficients. Although the principles of tomography were discovered in the first half of this century, it has been only recently that the availability of computing power has made commercial applications practical. Computed tomography was introduced as a diagnostic X-ray technology for medical applications in 1971; and, more recently, it has been applied to materials analysis, especially in the area of non-destructive evaluation. For example, CT scanners have been used to examine fiber-reinforced organic matrix composites, die castings, engine components, and tensile specimens for manufacturing anomalies, absolute density, density gradients, porosity, and dimensional inspections. The technique also has been used to characterize two-phase fluid flow through pipes and through porous media including visualization of laboratory core floods. Additional core data that can be obtained includes oil saturation, porosity, and mineral distribution.
One method for determining drilling mud invasion using CT is disclosed by Vinegar et al in U.S. Pat. No. 4,540,882. According to this method, a first material is added to the drilling fluid, i.e., the drilling mud, to obtain a first fluid that has either an effective atomic number that is different from the effective atomic number of the connate fluids in the rock formation or a density that is different from the density of the connate fluids, or both. A preserved core sample is collected from the borehole for scanning by a computerized axial tomographic scanner to determine the attenuation coefficients at a plurality of points in a cross-section of the core sample. The preserved core sample is scanned with the scanner at first and second energies, and the determined attenuation coefficients for the plurality of points in the cross-section at each energy are used to determine an atomic number image for the cross-section of the core sample. The depth of invasion of the first fluid is then determined from the atomic number image, as an indication of the depth of invasion of the drilling fluid into the core sample.
Another method of identifying drilling mud invasion is disclosed by Muegge et al in U.S. Pat. No. 4,722,095. In this method, reliance is had upon the high CT attenuation coefficient of barite that is commonly employed as a weighting agent in drilling mud. Initially, mud filtrate is removed from the core sample, after which measurements are made of the pore volume, bulk volume and mud solid volume of the core sample using CT.
Both of the above described methods focus upon the invasion of the drilling mud into the core sample. Neither method, however, distinguishes between drilling mud invasion and drilling mud filtrate invasion. The extent of drilling mud filtrate invasion into the core sample is important to know because the filtrate, which could be oil or water, needs to be differentiated from formation fluids in laboratory fluid saturation tests if accurate reservoir saturations are to be determined.
When using a drilling mud which forms a poor filter cake at the surface of the core sample, such as a barite weighted mud, there can be sufficient barite present in the filtrate to determine the extent of filtrate invasion. However, it is more likely that any solid particles in the mud will be held up or filtered out by small pore throats further inside the core if not by those at the core perimeter and that only "pure" oil or water filtrate will reach the full invasion extent.
A more serious problem in detecting filtrate invasion arises when drilling muds which form good filter cakes are used, such as a calcium carbonate mud, as is desired to limit the extent of invasion by drilling mud solids into the core sample. With these drilling muds the solids thereof are more effectively filtered out of the base fluid which then invades the core sample to a greater depth than the drilling mud solids. The filtrate, i.e., the filtered base fluid which enters the core, will be solid-free water or oil and cannot be differentiated from the connate fluids in the rock formation using conventional oil and water saturation techniques.
Still other problems arise when using some of the dopants identified by Vinegar et al in U.S. Pat. No. 4,540,882. To give a drilling fluid an effective atomic number different from the effective atomic number of the connate fluids, Vinegar et al lists as exemplary additives barium sulfate, calcium carbonate, sodium tungstate and sodium iodide. No distinction is drawn between drilling mud invasion versus drilling mud filtrate invasion nor between the use of any of these four additives in conjunction with different types of drilling muds or different coring conditions such as those encountered in deep boreholes. For example, barium sulfate and calcium carbonate fail as a filtrate dopant because they are both insoluble and as solid particles would be filtered out of the filtrate at the core surface or partway into the core, but not to the full invasion extent. A problem with sodium tungstate is that many connate fluids in reservoir rock formations contain magnesium which would cause sodium tungstate to precipitate and then be filtered out of the base fluid along with the other drilling mud solids.